Patent Literature 1, for example, discloses a conventional tag information processing apparatus etc. in which a plurality of tag readers provided in a distance from one another, so as to measure a time lag between readings of a tag ID of a tag by these tag readers, and estimate a movement direction of the tag based on the time lag.
Patent Literature 2 discloses another conventional tag information processing apparatus in which each receiving terminal keeps radio wave intensity of an RFID tag, and a movement direction of the RFID tag is judged based on a temporal change in value of the radio wave intensity.
Citation List
Patent Literature 1
Japanese Patent Application Publication, Tokukai, No. 2005-157698 A (Publication Date: Jun. 16, 2005) (see page 1 and FIG. 1, for example)
Patent Literature 2
Japanese Patent Application Publication, Tokukai, No. 2006-71466 A (Publication Date: Mar. 16, 2006) (see page 1 and FIG. 1, for example)
However, according to the tag information processing apparatus disclosed in Patent Literature 1, a movement direction of an RFID tag is estimated based on a time lag between a time at which a first tag reader reads a tag ID of the RFID tag and a time at which a second tag reader reads the tag ID. Accordingly, in a case where a read area of the first tag reader and a read area of the second tag reader overlap each other, the first and second tag readers simultaneously read a tag ID of an RFID tag located in an area where the two read areas overlap each other. That is, it is impossible to obtain a time lag between a time at which the first tag reader reads the tag ID and a time at which the second tag reader reads the tag ID. This causes a problem that a movement direction of a tag cannot be accurately judged.
Further, according to the tag information processing apparatus disclosed in the Patent Literature 2, a movement direction of an RFID tag is judged based on a temporal change in intensity of radio waves received from an area where read areas do not overlap each other and from an area where read areas overlap each other. This may make it impossible to read a tag ID of an RFID tag whose read rate is low even if the RFID tag is located in a read area. In this case, the tag information processing apparatus incorrectly judges that the RFID tag has moved out of the read area. As such, there occurs a problem that it is impossible to accurately judge a movement direction of a tag based on such a judgment result.